1. Field of the Invention
The present invention relates generally to an air-fuel ratio sensor that detects the air-fuel ratio of an engine. More particularly, this invention relates to a diagnostic apparatus that provides optimal air-fuel ratio control by detecting for failures which may occur in a temperature controller and heater circuit used to stabilize the temperature of the air-fuel ratio sensor.
2. Description of the Related Art
In the conventional motor vehicle engine arts, it is common to control the air-fuel ratio in order to optimize engine operating characteristics under various driving conditions. Such characteristics include the engine's output performance, exhaust content, and overall vehicle driveability. The air-fuel ratio control adjusts the amount of fuel supplied to the engine in accordance with the engine speed, the particular engine load, the warm-up status or the like in order to match the actual air-fuel ratio with a target value. An air-fuel ratio sensor, provided in the exhaust passage of the engine, is used as a component in this control to detect the actual air-fuel ratio.
An oxygen sensor is a typical air-fuel ratio sensor. One type of oxygen sensor is equipped with a heater 91 as shown in FIG. 9. This oxygen sensor uses an element 93, attached to a metal housing 92. The element 93 is shaped like a test tube, with the heater 91 disposed inside. A protection cover 94 covers the element 93. The element 93 can be made of annealed zirconia or titania and its outer surface is coated with porous platinum. The inner and outer surfaces of the element 93 are electrodes 95 and 96, respectively. Due to the exposure of the outer surface of the element 93 to the exhaust gas, a voltage is generated between both electrodes 95 and 96 in accordance with the oxygen density in this gas. This voltage is output from the oxygen sensor as a measure of the oxygen density.
The output characteristic of the element 93 depends in large part on the temperature of the exhaust gas. The element 93, when activated at a predetermined temperature, exhibits a stable output characteristic. The temperature of the exhaust gas around the sensor, however, varies depending on the particular driving condition of the engine as well as the location of the sensor in the exhaust passage. The oxygen sensor equipped heater 91 provides a way to compensate the output of the element 93 under various temperature conditions. The heater in this way maintains the temperature of the element 93 at a predetermined level.
In this type of heater-equipped air-fuel ratio sensor, the element temperature is optimally maintained by controlling the energization of the heater through an electric circuit.
The aforementioned temperature controller includes a heater and a wire harness connected to the heater. When this controller is used beyond its intended serviceability, however, it becomes prone to fail and may experience an electrical or mechanical disconnection. The conventional temperature controller is however provided with no means for detecting this type of failure. If the temperature controller does fail, the air-fuel ratio continues to be detected on the erroneous assumption that the temperature of the element is being optimally controlled. Consequently, the air-fuel ratio obtained from the control of the air-fuel ratio sensor may deviate from its target value, and ultimately deteriorate the exhaust emission of the engine.
One solution to the above described shortcoming is proposed in Japanese Unexamined Utility Model Publication No. 62-44272. This publication proposes an apparatus having a heater equipped air-fuel ratio sensor that can detect disconnections made in the temperature controller. The design of this detecting apparatus however utilizes a single air-fuel ratio sensor provided in the exhaust passage.
In this detecting apparatus, as shown in FIG. 10, a single heater 101, provided with the air-fuel ratio sensor, is connected in series to a driver 102. The heater 101, air-fuel ratio sensor and driver 102 form a heater energizing circuit. The driver 102 comprises a pair of Darlington-connected power transistors 103 and 104 and a plurality of resistors 105,106, 107, 108 and 109. An electronic control unit (not shown) inputs a control signal to an input terminal 111 of the driver 102. A battery (not shown) is connected to a power terminal 112 of the driver 102 so that the driver 102 and the heater 101 are connected in series to the battery. Depending on the engine's operating condition at any given time, as well as on the location of the air-fuel ratio sensor, the electronic control unit controls the driver 102 and the power supply to the heater 101 from the battery. The electronic control unit thus adjusts the element temperature of the air-fuel ratio sensor.
The above electric circuit incorporates a disconnection detector 113 responsive primarily to disconnections in the heater energizing circuit. This detector 113 comprises one comparator 114, two resistors 115 and 116 and a capacitor 117. One of the resistors, 115, is connected in series to the battery via the driver 102. The voltage drop across the resistor 115 is compared with a predetermined reference value by the comparator 114. When no voltage drop occurs across the resistor 115, a voltage of a predetermined level is output from an output terminal 118 of the detector 113. When engine conditions warrant the energization of heater 101, a predetermined voltage signal is output from the output terminal 118 of the detector 113. This allows the electronic control unit to determine that a disconnection exists in the heater energizing circuit. Following this, the electronic control unit outputs a signal energizing an alarm light inside the vehicle. This informs the driver of the disconnection existing in the heater energizing circuit.
The above disconnection detecting apparatus is designed to be used in an engine system that is equipped with just one air-fuel ratio sensor. If such an apparatus were to be used with a plurality of air-fuel ratio sensors, separate heater energizing circuits to would be needed to energize the heaters in the individual sensors. Each heater energizing circuit would in turn require a disconnection detector 113.
In a V shaped engine system, two exhaust passages may be provided in association with both banks of the engine. In such engines, a heater-equipped air-fuel ratio sensor would be provided in each exhaust passage. Alternatively, heater-equipped air-fuel ratio sensors could be provided at the upstream and downstream sides of a catalytic converter provided in an exhaust passage. In either case, a separate disconnection detector 113 would be required for each heater energizing circuit. The resultant design and overall structure of the disconnection detecting apparatus as described involves far too many component parts, is overly complex and impractical.
As mentioned above, the disconnection detector 113 functions mainly to detect whether disconnections exist in the heater energizing circuit. Such an apparatus, however, is not designed to detect failures other than such disconnections, e.g., performance degradation to the heater 101. Generally speaking, the heater wires or elements increasingly become thinner over long periods of service due to thermal deterioration or the like. Naturally, this degrades the heater's performance over time. Consequently, it would be desirable to detect the impaired performance of the heater 101 in order to allow its replacement when necessary.
One way to detect for deteriorations in the heater's performance is to control the current and temperature of the heater. To explain this, reference is made to the graph in FIG. 11 illustrating the relation between the heater's temperature and the current flowing through the heater (heater current). In this graph, the solid line characterizes a properly functioning heater, while the broken line characterizes a deteriorated heater. It is apparent from this graph that the heater current decreases as the heater temperature rises. This is due to the increasing resistance of the heater as the heater temperature rises.
When the diameter of the heater decreases due to thermal deterioration, the resistance of the heater increases while the amount of heater current decreases. This is illustrated in FIG. 11 by the change in the heater's characteristic from the solid to broken line. Such a deterioration in the heater's performance can be detected by determining the value of the heater current at a certain heater temperature. In consideration of the variously produced and manufactured heaters, as well as the difference in heater temperatures at the time the heater is energized, diminishing heater efficiency or heater degradation may be detected in the following manner.
A reference value must be set to allow for a comparison with the value of the heater current. This reference value is often set to a relatively low level (e.g., 0.1 A) as indicated by a lower alternate long and short dash line L in FIG. 11. In such a case, however, the range over which deteriorated heater performance can be detected narrows considerably with increasing heater temperatures. Moreover, the current value is limited to a relative small range (e.g., 0.1 to 0.2 A). This degrades the precision with which heater performance can be detected. To more accurately detect the deterioration in heater performance based on the value of the heater current, the reference value for comparison should be set to a relatively high level (e.g., 0.2 A) as indicated by an upper alternate long and short dash line H shown in FIG. 11. This would eliminate the effects or influence of the increased heater temperature on the detection process.
To accurately detect reductions in heater performance, without setting the reference value disadvantageously low, the heater may be temporarily deactivated during those periods of time when heater control is undertaken. This would allow the values for the heater's current and voltage to be determined at a time when the heater temperature are low.
With this method, however, when the engine is actually undergoing the air-fuel ratio control, the heater can only occasionally be deactivated. This effectively reduces the number of times which the conventional air-fuel ratio control operates to detect deteriorated heater performance. Moreover, the validity of this method is suspect due to the fact that actual heater performance control or detection may not be performed for long periods of time.